no additional plugin is needed, to save as jpg, if gimp was correctly installed is only needed to add .jpg after the image name in the Save AS or " Save Copy as " dialogs
(at least if you did not change default save option)
As example if your want call your image "my-image" to save as jpg you just wrote there " "my-image.jpg "
If then you get message about missing plugin then is probable you downloaded a corrupt installer

At least with Gimp 2.8 on a Macbook Pro, you can't actually "save" an image as a jpg (or anything other than a .xcf). You have to go to File->Export and "export" the image to a non-gimp format. I hope the developers can fix this.